Shock resistant horn antenna



Augl, 1957 E. J. WILKINSON 3,334,347

SHOCK RESISTANT HORN ANTENNA Filed April s, 1964 5 sheets-sheet 1 F/G. l

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ATTORNEY United States Patent 3,334,347 SHOCK RESISTANT HORN ANTENNAErnest J. Wilkinson, Westwood, Mass., assignor to Syl- Vania ElectricProducts Inc., a corporation of Delaware Filed Apr. 9, 1964, Ser. No.358,581 Claims. (Cl. 343-719) This invention relates to antennas, andmore particularly to ultra high frequency antennas which can functionunder severe environmental conditions.

. It has become increasingly important for military purposes to providecommunication systems which are hardened; that is, which are capable ofsurviving the extreme environment encountered during a nuclearexplosion, and which can function substantially unimpaired after such anexplosion. It is a relatively simple matter to protect the electronicequipment itself from the effects of an explosion by locating theequipment in protective underground shelters. It is a more diliicultproblem, however, to protect the antenna which must be open to theatmosphere to remain operational. There are three major obstacles thatmust be overcome in designing a hardened antenna. The most obvious isthat the antenna must have the structural strength to withstand theforce of an explosion, such as caused by a nuclear bomb. Another is thatthe antenna must withstand the high temperatures associated with thefireball. Thirdly, the antenna must function in vsurrounding debris,such as dirt, stones, and the like, which may be conductive and whichmay completely or partially bury the antenna. It is evident that anytype of antenna which protrudes above the surface of the ground, eveniff it withstands the shock wave and thermal effects of an explosionwould be of little use as an antenna since conductive debris whichshowered the antenna would in all probability short circuit it toground. It is, therefore, an object of the present invention to providean eicient, relatively inexpensive, hardened antenna which offers a highdegree of protection against shock and thermal effects associated withan explosion, and which, in particular, is not affected by debrisaccumulation.

Another object of the invention is to provide a iiush mounted antennaelectrically equivalent to a stub antenna, this type of antenna havingradiating characteristics that have Ibeen found to be particularlysuitable for UHF` communication systems.

. A nuclear explosion can cause overpressures outside of the craterregion of greater than 1000 p.s.i., and temperatures of 30,000" C. inthe vicinity of the fireball. It would be expected that a formidablestructure would be required to survive such conditions, and that itwould be an even more formidable task to provide an antenna structurethat would be operational after su-ch an explosion. In accordance withthe present invention, however, a relatively simple structure isutilized to provide both the requisite mechanical strength andelectrical operation. Briefly, the antenna comprises a hollowcylindrical pipe flush mounted in the ground which is excited in the TMmmode by means to be described hereinafter, the region `at the bottom ofthe pipe containing no electromagnetic field. Debris caused, forexample, by a ibomb blast, falling into the pipe will collect at thebottom, and, therefore, not interfere with the electri-cal operation ofthe antenna. Moreover, the antenna structure being below ground, willnot receive the brunt of the shock from an explosion. Also, thetemperature at the feed point, located about half way down the pipe,will be greatly reduced relative to the temperature at the surface dueto the ifact that thermal and nuclear radiation cannot bend around acorner.

The foregoing, and other objects, features, and advantages of thepresent invention, together with a better understanding of itsconstruction and operation, will be more apparent from 'the followingdetailed description,

3,334,347 Patented Aug. 1, 1967 taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a sectional elevation view of a preferred embodiment of theinvention;

FIG. 2 is a fragmentary sectional elevation view showing an alternativemeans of energizing the present antenna;

FIG. 3 is a sectional elevation view of another embodiment of theinvention;

FIG. 4 is a pictorial view of a corner reflector useful to convert theantenna into4 a high gain directional antenna;

FIG. 5 is a pictorial view of a further embodiment of the invention;

FIG. 6 is a plot of the measured VSWR characteristic of the embodimentof FIG. 1;

FIG. 7 is a plot of the measured elevation pattern of the embodiments-of FIGS. 1 and 3; and

FIG. 8 is a plot of the measured elevation pattern of the embodiment ofFIG. 4.

Referring` to FIG. l, the illustrated preferred embodiment of theinvention comprises a cylindrical conductive pipe 10, typically made ofsteel, rigidly supported in the ground 12 with its aperture flush withthe surface of the ground. The pipe has a middle region 14 of uniformdiameter, the top end of which tapers into a region 16 of largerdiameter, and the bottom end of which tapers into a region 18 of smallerdiameter. The middle region 14 constitutes the antenna and is of asuitable diameter to propagate the specified electromagnetic eld. Theenlarged upper region 16 provides impedance matching between the antennaand the atmosphere while the smaller region 18 is of such a Idiameterthat this region of the pipe is cut olf at the operating frequency, forreasons to be subsequently explained.

The structural strength required in a particular instance will of coursedictate the necessary wall thickness of pipe 10. Calculations have shownthat a pipe thickness of approximately four inches should be adequate towithstand the forces and temperatures expected from a nuclear explosion.To further support the antenna, various well known constructiontechniques can be employed, such as imbedding pipe 10 in concrete. Thefrequency at which the antenna operates will, of course, govern itsdimensions. For an antenna designed to operate at 300 megacycles, forexample, regions 14, 16 and 18 of pipe 10 had inside diameters of 3feet, 4 feet and 2.5 feet, respectively. The overall length of pipe 10is 12 feet.

The antenna is energized by a pair of feeds 20 and 22 mounted atdiametrically opposite positions in the lower end of region 14. Thefeeds each consist of a rectangular cavity formed by conductive wall 24having an open face llush with the inner wall of pipe 10. Each cavity isexcited by a suitably placed probe 26 provided within the cavity, oralternatively by loops or other well known means. Each cavity is filledwith a dielectric material 28, such as epoxy-loaded Fiberglas, toprotect the feed probes from the effects of a shock wave that may bepropagated within pipe 10. Since the antenna feed structure is liushmounted in the walls of pipe 10, shock waves propagated down the pipepass over the feed without damaging it. Moreover, hot gases presentabove ground will not significantly affect the antenna structure sinceonly thermal radiation from gases colinear with the antenna axis canenter the pipe to raise the temperature in the region of the feed.

For ease of explanation the operation` of the invention will bedescribed in the transmitting mode, but it is to be understood that itwill function equally well in the receiving mode. In operation, energyfrom a suitable energy source 36 is applied in phase via equilengthcables 30 and 32 and respective probes 26 to corresponding cavities 24which, in turn, excite a TMm mode electromagnetic field within portion14 of pipe 10. Since this field configuration is the same as that of astub antenna, an antenna pattern is produced which is identical to thatof a stub antenna.

The diameter of region 1S being dimensioned beyond cutoff at theoperating frequency, no energy propagates into this region, and it isclear that foreign matter that may be located withinpthis region willnot affect antenna performance. Thus, debris from a bomb explosion, orother cause, falling into pipe will accumulate in the bottom of the pipewhere no field exists and will not in any way detract from theperformance of the antenna. The length of region 18 can be suitablychosen to accommodate the expected amount of debris. Debris accumulatingaround the lip of the pipe aperture has been found to have a negligibleeffect on antenna patterns or impedance, since the principal fieldsabout the antenna are those within the aperture itself.

Although the invention is especially attractive as a hardened antenna,it is by no means limited to such use but can also be employed in moreconventional applications either as a tracking antenna or as a monopulsefeed for a parabolic refiector or lens. It will be recalled that theantenna radiates in the TMm mode when feed probes 26 are energized inphase. If, however, the probes are energized in opposite phase, the TEUmode is excited, with the resulting antenna pattern being normal to theaperture. It will be appreciated that this broadside pattern has maximumintensity on the antenna axis, while the TMm mode produces a patternhaving a minimum intensity or null on the antenna axis. These patternswill be recognized as the familiar sum and difference patterns of amonopulse tracking antenna. The present invention, therefore, has thecapability of providing monopulse information simply by switching theexcitation from inphase to opposite-phase. One means by which thisswitching can be accomplished is shown in FIG. 2, wherein probes 26 areconnected via cables 30 and 32 to a hybrid junction 40 havinga sum port42 and a difference port 44. When sum port 42 is energized by a signalfrom a signal source, the probes are fed in phase and a differencepattern results; conversely when difference port 44 is energized, theprobes are fed 180 out of phase causing a surn pattern to be produced.The antenna operating in this manner is particularly useful as amonopulse feed for a parabolic refiector since it is hollow and would,therefore, not interfere with optical alignment of the refiector. It canalso be used directly mounted in the ground as shown in FIG. l, fordeveloping rough elevation tracking information in hardened or otherapplications.

Another embodiment of the invention is illustrated in FIG. 3, andconsists of a conductive cylindrical pipe 50 of uniform cross-section,except for an aperture portion 52 of enlarged diameter which functionsas an irnpedance transformer, as in the embodiment of FIG. 1.Diametrically disposed, at a position approximately midway of the lengthof pipe 50', are an upper pair of energizing feeds 54 and 56, and alower pair of feeds 58 and 60, separated from the upper pair by adistance equal to a quarter wavelength at the center frequency ofoperation. Each feed comprises a dielectric loaded cavity energized by aprobe suitably placed within the cavity as described hereinabove. Upperfeeds 54 and 56 are energized by probes 62, while lower feeds 58 and 60are energized by probes 64. Probes 62 are connected by equal lengthcables 66 and 68 to an energy source 70 via a T-junction 72. Probes 64are connected by equal length cables 74 and 76 through T-junction 72 toenergy source 70. The length of arm 78 of T-junction 72 is one quarterwave longer than arm 80. A signal applied via arm 78 to probes 64 will,therefore, lag by 90 the signal applied to probes 62. Since the upperand lower feeds are displaced from each other by a quarter wavelength,as measured in the guide, it is evident that the fields produced by eachpair of feeds will combine in phase in the region adjacent the upper-feeds and above, while these fields will cancel in the region below thelower feeds. The region below the lower set of feeds, therefore,contains no electromagnetic field and debris falling into this regiondoes not affect the antenna performance.

This embodiment is more frequency sensitive than the embodiment depictedin FIG. 1, since the field cancellation depends upon precise relativephasing of the energizing feeds and by the accuracy of the quarter wavespacing between the two sets of feeds. In practice, about 99%cancellation of the field in the region below feeds 58 and 60 has beenattained. This embodiment, however, has some mechanical advantage overthat of FIG. 1 in that the walls of the pipe are straight throughout itslength, except for the impedance matching section; thus, there are noprotuberances that would receive the force of a shock wave that maypropagate down the pipe.

To give the antenna a directional pattern, a suitable refiector, such asthe corner reector illustrated in FIG. 4 may be used in conjunction withthe present antenna. As shown in FIG. 4, the corner reflector 80consists of a conductive plate 84 supported by a block of concrete 86 orother material to provide structural strength capable of withstanding anexplosive blast. It will be remembered that the present antenna iselectrically equivalent to a stub antenna, as indicated by the fieldconfiguration illustrated in the aperture 82; therefore, any type ofrefiector usable with a stub antenna can be similarly employed with thepresent antenna.

The present invention can also be embodied in a rectangular duct,illustrated in FIG. 5, which is excited in the T M11 mode to produce abi-directional pattern in the azimuth plane. This embodiment consists ofa rectangular conductive duct 90, which can be mounted vertically in theground in the same manner as described hereinbefore, and which isenergized by an upper pair of feed structures 92 and 94 disposed onopposite side walls of duct 90, and a lower pair of feed structures 96and 98 also disposed on respective side walls and vertically disposedone quarter wavelength from feeds 92 and 94. Each feed structure issimilar to that described hereinabove, and consists of a rectangularcavity open to the inside of duct 90; however, in this instance eachcavity is fed by a plurality of probes 100 uniformly spaced within thecavity to provide the requisite energization to produce the TMllradiation mode. This ernbodiment has higher gain than that of thecircular pipe embodiments since a bi-directional azimuth pattern isproduced, rather than the omnidirectional azimuth pattern of thecircular pipe antenna. A unidirectional antenna pattern can be providedby placing a vertical planar reflector parallel to one side wall.

The performance of antennas constructed in accordance with the inventionis illustrated in FIGS. 6, 7 and 8. FIG. 6 shows the measured VSWR plotof an antenna of the type shown in FIG. 1. The solid curve is the VSWRof the antenna operating without debris, while the dotted curve 112 isthe VSWR when the antenna aperture is surrounded by debris. It can beseen that a VSWR of 2.4 or less is maintained over a twenty percentfrequency band, and that debris around the aperture has a minimal effecton impedance. No measurable change in impedance could be detected due todebris located in the bottom of the antenna pipe. It is evident, then,that antenna performance is not degraded by the presence of debrisaround the aperture or in the antenna pipe.

FIG. 7 depicts the measured elevation pattern of an antenna of the typeshown in FIGS. 1 or 3. The diameter of the aperture of the measuredantenna was approximately three-quarters of a wavelength. The antennapattern is identical for either embodiment since the pattern isdetermined by the field configuration within the aperture regardless ofthe energizing means by which this field configuration is produced. Itwill be noted that this antenna pattern closely resembles that of a stubantenna, as mentioned hereinabove.

The directional elevation pattern of an antenna of the type depicted inFIG. 4 is shown in FIG. 8, and is seen to be essentially one-half of theomnidirectional pattern of FIG. 7 but having higher gain.

From the foregoing, it is evident that a simple, broadband antenna hasbeen provided which can withstand the extreme environmental conditionscaused by a nuclear explosion and which can function unimpaired in thepresence of debris.

While there has been shown what are now thought to be preferredembodiments of the present invention, various alternatives will occur tothose versed in the art without departing from the true spirit and scopeof the invention. Accordingly, it is not intended to limit the inventionby what has been particularly shown and described, except as indicatedin the appended claims.

What is claimed is:

1. An antenna capable of operating under severe environmental conditionscomprising, a conductive cylindrical pipe of uniform inside diameter andhaving an impedance `matching aperture section on one end thereof, afirst pair of energizing feeds diametrically disposed in the wall ofsaid pipe, a second pair of energizing feeds diametrically disposed inthe wall of said pipe and axially displaced from said first pair offeeds by a distance equal to a quarter guide wavelength, means forapplying energy to said first pair of feeds, and means for applyingenergy to said second pair of feeds, said last mentioned energy being inphase quadrature with respect to said energy applied to said first pairof feeds.

2. The antenna according to claim 1, wherein each energizing feedcomprises a dielectric lled rectangular cavity having an aperture enddisposed iiush with the inside Wall of said cylindrical pipe.

3. An antenna capable of operation under severe environmental conditionscomprising, a conductive duct of uniform cross section and having animpedance matching aperture section on one end thereof, a first pair ofenergizing feeds respectively disposed in opposite side walls of saidduct, a second pair of energizing feeds respectively disposed in saidside walls and axially displaced from said rst pair of feeds by adistance equal to a quarter guide wavelength, means for applying energyto said first pair of feeds, and means for applying energy to saidsecond pair of feeds, said last-mentioned energy being in phasequadrature with respect to said energy applied to said first pair offeeds.

4. An antenna capable of operation under severe environmental conditionscomprising, a c-onductive cylindrical pipe having a central portion of adiameter which can sustain a TMm electromagnetic field, an impedancematching aperture region on one end of said pipe of a diameter largerthan said central portion, a taper section connecting said apertureregion and one end of said central portion, a region of smaller diameterthan said central portion on the other end of said pipe dimensioned tobe incapable of sustaining said TMm field, a taper section connectingsaid region of smaller diameter and the other end of said centralportion, first and second dielectric filled feed cavities diametricallydisposed in the wall of said central portion, each having an apertureend iiush with the inside wall of said central portion, first and secondfeed probes disposed Within respective cavities and operative to coupleenergy thereto or therefrom, and means for coupling energy to or fromsaid feed probes.

5. An antenna capable of operating under severe environmental conditionscomprising, a conductive cylindrical pipe having a central portion of adiameter which can sustain a specified electromagnetic field, animpedance matching aperture region on one end of said pipe of a diameterlarger than said central portion, a taper section connecting saidaperture region and one end of said central portion, a region of smallerdiameter than said central portion on the other end of said pipedimensioned to be incapable of sustaining said specified field, a tapersection connecting said region of smaller diameter and the other end ofsaid central portion, first and second feed cavities diametricallydisposed in the wall of said central portion, each having an apertureend ush with the inside wall of said central portion, and means forcoupling energy to or from said cavities.

References Cited UNITED STATES PATENTS 2,343,531 3/ 1944 Buchholz343-786 2,369,808 2/ 1945 Southworth 343-719 X 2,398,096 4/1946 Katzin343-786 X 2,587,055 2/1952 Marshall 33.3-10 X 2,748,350 5/1956 Miller333-10 2,809,371 10/1957 Carter et al. 343-786 3,267,475 8/1966 Howard343-786 X ELI LIEBERMAN, Primary Examiner.

CHESTER L. JUSTUS, Examiner.

R. E. BERGER, Assistant Examiner.

5. AN ANTENNA CAPABLE OF OPERATING UNDER SEVERE ENVIRONMENTAL CONDITIONSCOMPRISING, A CONDUCTIVE CYLINDRICAL PIPE HAVING A CENTRAL PORTION OF ADIAMETER WHICH CAN SUSTAIN A SPECIFIED ELECTROMAGNETIC FIELD, ANIMPEDANCE MATCHING APERTURE REGION ON ONE END OF SAID PIPE OF A DIAMETERLARGER THAN SAID CENTRAL PORTION, A TAPER SECTION CONNECTING SAIDAPERTURE REGION AND ONE END OF SAID CENTRAL PORTION, A REGION OF SMALLERDIAMETER THAN SAID CENTRAL PORTION ON THE OTHER END OF SAID PIPEDIMENSIONED TO BE INCAPABLE OF SUSTAINING SAID SPECIFIED FIELD, A TAPERSECTION CONNECTING SAID REGION OF SMALLER DIAMETER AND THE OTHER END OFSAID CENTRAL PORTION, FIRST AND SECOND FEED CAVITIES DIAMETRICALLYDISPOSED IN THE WALL OF SAID CENTRAL PORTION, EACH HAVING AN APERTUREEND FLUSH WITH THE INSIDE WALL OF SAID CENTRAL PORTION, AND MEANS FORCOUPLING ENERGY TO OR FROM SAID CAVITIES.